"The data suggest that a 20-pound increase in men's weight may increase the chance of infertility by about 10 percent," says Markku Sallmen, lead author on the paper who is now at the Finnish Institute of Occupational Health. Body mass index (BMI) provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems.

The researchers studied couples enrolled in the Agricultural Health Study (AHS), a large project that began in 1993 examining factors that impact the health of farmers and their families in agricultural communities.

"Women who are overweight or obese tend to have a more difficult time becoming pregnant than normal-weight women, but whether men who are overweight or obese also have fertility problems had not been studied," says Donna Baird, Ph.D., an National Institute of Environmental Health Sciences (NIEHS) epidemiologist with the study.

Researchers found that men's BMI was an independent risk factor for infertility. The researchers adjusted for other factors that could affect fertility, including high BMI of the woman, age, cigarette smoking, alcohol intake, and solvent and pesticide exposure. After adjustment, there was a general increase in infertility with increased BMI, reaching a nearly 2-fold increase among obese men.

When researchers divided the sample into two equal groups by men's age, they found that men's BMI was a risk factor for infertility in both the older and younger men.

The researchers did not have data on frequency of sexual intercourse, so it is possible that overweight men have less sexual intercourse than their normal weight counterparts and this could influence fertility. However, there have been recent studies looking at semen characteristics that show lower semen quality for overweight and obese men, as well as hormonal differences.